Air conditioning units for motor vehicles typically include a compressor, a heat exchanger delivering heat, i. e. a condenser, or gas cooler, respectively, an expansion element, and one or several heat exchangers absorbing heat, for example evaporators. The heat-delivering heat exchanger functions to cool the refrigerant after having been compressed and heated as close as possible to the ambient temperature level. The heat-delivering heat exchanger is typically positioned in the engine compartment between the head lamps exposed to the air stream. When the air stream is insufficient, functioning of the heat-delivering heat exchanger is additionally supported by one or several fans.
Traditional air conditioning units are usually passed by a refrigerant that frequently has a composition including fluorine. Such units are, as a rule, operated in the sub-critical mode. That means that the diphase region of the refrigerant is passed when the refrigerant is evaporated at the low temperature level, as well as when the refrigerant is condensed at the high temperature level. Further, air conditioning units are known that function based on natural refrigerants, such as carbon dioxide. A refrigeration circuit with carbon dioxide as refrigerant functions, as a rule, in the supercritical mode. This means that in the refrigerant condition obtained by the compression, both the temperature and the pressure of the compressed carbon dioxide are higher than the critical pressure and critical temperature of carbon dioxide. Therefore, during subsequent delivery of heat by the refrigerant in the gas cooler, the wet vapor region is not passed.
It is decisive for the refrigerating capacity of a motor vehicle air conditioning unit, particularly in compression refrigeration circuits operated above supercritically, to achieve a refrigerant temperature as low as possible before the refrigerant is fed to the expansion element. To this end, circuit designers usually attempt to approximate the refrigerant temperature in the heat-delivering heat exchanger as close as possible to the ambient temperature.
In many vehicles, it is difficult to make such an approximation to the ambient temperature, particularly during a halt of the vehicle or under unfavorable ambient conditions. On the cooling airside, for example, due to heated pavement surface or backflow from the engine compartment, the input temperature of the cooling air may be significantly higher than the ambient temperature. This undesired preheating of the cooling air may often even be by 20 to 25 degrees centigrade. Also oil coolers and charge-air coolers positioned upstream contribute to this effect. Even if the temperature of the refrigerant was lowered to the temperature of this relatively hot cooling air, which is virtually impossible by mere heat transfer, the resulting cooling effect, hence the refrigeration capacity, would be insufficient in many cases. As a consequence, depending on the circuit design, the optimal high pressure or the necessary refrigeration capacity, respectively, can no longer be realized. A decrease in the COP and an increase in the power demand of the compressor drive result. In particular, at high ambient temperatures, where a high refrigeration capacity is normally required, these effects are markedly negative, not compatible with the comfort demands as having become usual in the meantime.
It is known to increase the capacity of an air conditioning unit for motor vehicles by passing the refrigerant through branched flow paths, for example, using an internal heat exchanger with separated flows upstream of an evaporator. Documents WO 2005/059449A1 and DE 10060114A1 are illustrative of such a design. However, such a design places relatively high installation demands for the realization of the various flow paths and the great number of line connections.
Further, it is known to cool the refrigerant before it is fed to a condenser/gas cooler. This, for example, can be obtained in that between the compressor and the throttling device of an air conditioning unit an additional heat exchanger is provided that extracts heat from the compressed refrigerant before the refrigerant reaches the condenser/gas cooler. Document DE 10231645A1 is illustrative of such a design. In this way, the inlet temperature on the hot side of the condenser/gas cooler is reduced. However, the problem of a too high ambient temperature as the lowest theoretically achievable refrigerant temperature at the outlet of the heat-delivering heat exchanger remains unsolved with this solution.
It is further known to reduce the temperature of the refrigerant before the inlet into an evaporator by that before the evaporator a heat exchanger is installed where heat is extracted from the refrigerant in countercurrent mode by evaporating liquid water. For this, the water to be evaporated is supplied from the collected condensate of the air conditioning unit and fed to the heat exchanger. Document DE 10159148A1 is illustrative of such a design. This solution is disadvantageous in that the formation of condensate and the efficiency of the evaporation strongly depend on the ambient conditions, particularly the humidity of the air. Hence the supporting function of such an assembly can only be controlled within narrow limits. Furthermore, functioning of this additional cooling depends on the operational state of the air conditioning circuit. At least a certain operational time in advance is necessary until a sufficient condensate flow will have started to supply the additional heat exchanger with condensate.
It is also known to include in the refrigerant circuit a cold store switched in series that, especially when the refrigerating capacity of the evaporator is not sufficient or not available, provides a cooling potential. Document DE 10258618B3 is illustrative of such a design. Such systems are preferably used for the air conditioning of motor vehicles at rest, serving to cool the passenger cell before the compression circuit of the real air conditioning unit has developed its full effect.
It would be desirable to produce an air conditioning unit for a vehicle that provides sufficient refrigerating capacity at a maximized efficiency under critical conditions such as at high ambient temperatures and periods when the vehicle is stationary.